This invention relates to apparatus and techniques for making nuclear magnetic resonance (NMR) measurements in boreholes, and to methods for determining magnetic characteristics of formations traversed by a borehole. Specifically, the invention relates to an NMR tool that has a large region of investigation within the formation as a result of measuring NMR spins at frequencies determined by the earth""s magnetic field.
A variety of techniques have been used in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) or logging-while-drilling (xe2x80x9cLWDxe2x80x9d). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (xe2x80x9cMWTxe2x80x9d).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as xe2x80x9cT1xe2x80x9d), and transverse relaxation time (generally referred to as xe2x80x9cT2xe2x80x9d) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B0 in the vicinity of the wellbore, and an oscillating magnetic field B1 in a direction perpendicular to B0. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B0 field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B1 field is to rotate the magnetic moments by an angle xcex8 controlled by the width tp and the amplitude B1 of the oscillating pulse. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (xe2x80x9cCPMGxe2x80x9d) sequence that can be expressed as
TWxe2x88x9290xe2x88x92(txe2x88x92180xe2x88x92txe2x88x92echo)nxe2x80x83xe2x80x83(1) 
where TW is a wait time, 90 is a 90 degree tipping pulse, and 180 is a 180 degree refocusing pulse.
After being tipped by 90xc2x0, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency xcfx890, given by xcfx890=xcex3B0, where B0 is the field strength of the static magnetic field and xcex3 is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the xe2x80x9cspin-lattice relaxation timexe2x80x9d or T1. Inhomogeneities of the B0 field result in dephasing of the magnetic moments and to remedy this, a 180xc2x0 pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals. These echo sequences are then processed to provide information about the relaxation times.
U.S. Pat. No. 4,350,955 to Jackson et al discloses a pair of permanent magnets arranged axially within the borehole so their fields oppose, producing a region near the plane perpendicular to the axis, midway between the sources, where the radial component of the field goes through a maximum. Near the maximum, the field is homogeneous over a toroidal zone centered around the borehole. With the Jackson arrangement, the axial extent of the region of examination is quite limited. As a result of this, the device can only be operated at relatively low logging speeds: otherwise, because of the tool motion during logging, the magnitude of the static field changes significantly within a fixed region of the formation with an accompanying degradation of NMR signals.
There are several devices in which the problem of limited axial extent of the basic Jackson configuration of permanent magnets is addressed. U.S. Pat. No. 4,717,877 to Taicher et al teaches the use of elongated cylindrical permanent magnets in which the poles are on opposite curved faces of the magnet. The static field from such a magnet is like that of a dipole centered on the geometric axis of the elongated magnets and provides a region of examination that is elongated parallel to the borehole axis. The RF coil in the Taicher device is also a dipole antenna with its center coincident with the geometric axis of the magnet, thereby providing orthogonality of the static and magnetic field over a full 360xc2x0 azimuth around the borehole.
U.S. Pat. No. 6,023,164 to Prammer discloses a variation of the Taicher patent in which the tool is operated eccentrically within the borehole. In the Prammer device, NMR logging probe is provided with a sleeve having a semi-circular RF shield covering one of the poles of the magnet: the shield blocks signals from one side of the probe. The probe is provided with elements that press the uncovered side of the probe to the sidewall of the borehole so that signals from the uncovered side arise primarily from the formation.
A basic problem with currently used NMR logging tools and methods is that they are based on the generation of a strong magnetic field in the earth formation and operate at frequencies of the order of 1 MHz. The region of investigation is typically no more than a few centimeters inside the formation due to the decay of the static field away from the permanent magnet. A stronger magnetic field, besides being harder to generate, is self-defeating as this pushes up the Larmor frequency to a point where eddy current and dielectric dissipation of the NMR signals become significant. For some tools, the thickness of the region of investigation is of the order of a few millimeters at best because of the spatial variability of the magnetic field. As a result of this, logging is susceptible tool movement in direction perpendicular to borehole axis. For other tools the length of region of investigation along the longitudinal axis is short. As a result of this, logging speed is restricted.
The lack of penetration into the formation means that if a wireline is used for the NMR logging, the logging must be done a short time after drilling to avoid the development of a mudcake. Even if the wireline logging is done a short time after drilling or if a Measurement-while-Drilling method is used, there is still the possibility of invasion of the formation by drilling fluids, so that the logging tool does not measure properties of the virgin formation. Additionally, the drilling process itself usually alter rock formation properties in a close vicinity of a borehole by causing some mechanical damage.
An alternative approach is to use the earth""s magnetic field for providing the static field for NMR measurements. The Larmor frequency f for protons at the earth""s magnetic field is approximately 2.5 kHz. The signal level per unit volume for an NMR survey is approximately proportional to f7/4 which means that the signal per unit volume at 2.5 kHz is reduced to about 25xc3x9710xe2x88x926 of the value for 1 MHz. However, the earth""s magnetic field is uniform everywhere an NMR signal can be generated and received, so that the loss of signal intensity is more than made up by receiving a signal from a much larger volume: this makes it feasible to perform NMR logging wherein the precession frequency is determined by the earth""s magnetic field.
U.S. Pat. No. 3,004,212 to Coolidge et al discloses a combined NMR and induction logging tool. An electrical current is passed through a polarizing coil in the shape of an elongated rectangle to produce a polarizing field that is perpendicular to the longitudinal axis of the borehole. This aligns the nuclear spins in the formation parallel to the polarizing field which will be inclined to the earth""s magnetic field. Upon switching off the electrical current within a period of 1-10 ms., precession of the nuclear spins about the earth""s magnetic field occurs. The signals resulting from the precession may be detected either in the polarizing coil or in a second coil orthogonal to the polarizing coil. Coolidge also teaches the use of stepping down the electrical current from a higher level to a lower level to establish different relaxation conditions on the precession as well as changing the duration of the lower current to plot a rate curve for the nuclear polarization.
U.S. Pat. No. 3,188,556 to Worthington teaches the introduction of a paramagnetic material such as molecular oxygen that is soluble in water to distinguish between the relaxation of protons in an oil phase and of protons in a water phase.
U.S. Pat. No. 3,538,429 to Baker discloses NMR logging in the earth""s magnetic field wherein the polarizing coils are configured to suppress 60 Hz. noise without adversely affecting the precession signals from the formation.
U.S. Pat. No. 2,999,204 to Jones et al discloses the use of passing a compensating current in the reverse direction of the polarizing current through the polarizing coil after the polarizing current has been turned off: by properly selecting the magnitude and/or time of the compensating current, it is possible to compensate for dephasing of the precession due to inhomogeneities in the earth""s magnetic field. Jones discusses the possible causes of the inhomogeneities as being magnetically retentive material in the earth formations and in the drilling fluid within the borehole. Dephasing of the precession due to magnetic field inhomogeneities causes the magnetic induction signal to decay faster as compared to homogeneous magnetic field.
U.S. Pat. No. 3,042,855 to Brown addresses another problem in NMR logging in the earth""s field, namely the strong precession signal produced by borehole fluids. Brown teaches the use of a hollow plastic bag or an expandable bag for enclosing the polarizing and detecting coils with the space around the coil filled with a flowable non-magnetic material. U.S. Pat. No. 4,035,718 to Chandler discloses an alternative arrangement in which the polarizing and detecting coils are enclosed within a nonmagnetic housing containing a colloidal suspension of ferromagnetic particles.
A fundamental problem with the above described teachings is the fact that due to local magnetic field inhomogeneity the precession signals dephase relatively rapidly, without any possibility of forming an echo, making the signals harder to interpret. In addition, there is an additional problem of a dead time following the very fast non-adiabatic turning off the polarizing current during which measurements are dominated by noise. The present invention addresses these problems.
The present invention is an apparatus and method for performing nuclear magnetic resonance measurements using a borehole tool. A polarizing coil on the tool is activated to align nuclear spins in the earth parallel to the magnetic field produced by the coil. The current in the polarizing coil is turned off adiabatically. The nuclear spins that were aligned parallel to the induced field realign in a direction of earth""s magnetic field and precess about the earth""s magnetic field with a Larmor frequency corresponding to the earth""s field. This frequency is within the audio frequency (AF) band at about 2.7 kHz. A second coil is pulsed with a CPMG sequence (or other suitable sequence) to perform spin echo measurements on precessing spins. The second coil may be used as a receiving antenna for the purpose. Additionally, a third coil may be used for detecting the spin echo signals.
The method may be used in inclined or vertical boreholes. The duration of the polarizing pulse is several times the largest value of T1 of the formation, typically 6 seconds. The turning off of the polarizing pulse is done over a time interval of 100 ms. or more. When two antenna coils are used, the AF signal may be beam-steered to have a direction that is transverse to the earth""s magnetic field.